The present invention relates to a bootstrap circuit, and more specifically to a bootstrap circuit which is used in a MOS (metal oxide semiconductor) integrated circuit such as a circuit for generating clock pulses to drive word lines.
A bootstrap circuit used in a MOS integrated circuit is constructed, and mentioned below and has been disclosed, for example, in the U.S. Pat. No. 3,744,055. Namely, a load MOS transistor and a drive MOS transistor are connected in series between a high potential source and a low potential source thereby forming an inverter, and a capacitor is connected between the point at which the above-mentioned two transistors are connected together and the gate of the above-mentioned load transistor. Further, a circuit for electrically charging the capacitor and a circuit for discharging the capacitor are connected to the terminal of the capacitor which is connected to the gate of the load transistor. The discharge circuit includes a MOS transistor that is connected between the terminal of the capacitor connected to the gate of the load transistor and the above-mentioned low potential source. The discharge circuit is rendered conductive to discharge the above-mentioned capacitor when a reset signal is applied to the gate of the MOS transistor.
In the bootstrap circuit constructed as mentioned above, the potential at the terminal of the capacitor that is connected to the gate of the load transistor before the discharge takes place is higher than the potential of the high potential source. Therefore, a high voltage is impressed between the drain and the source of the MOS transistor in the discharge circuit during the initial stage of the discharge, while the potential of the reset signal applied to the gate is smaller than that of the high-potential source. Consequently, the MOS transistor operates in a deeply saturated region. When a MOS transistor operates in a deeply saturated region, in general, hot electrons of high energy are generated in large amounts and are captured by the gate oxide film, causing the threshold voltage to undergo variation. Even putting aside the generation of hot electrons, the flow of current in the deeply saturated region brings the transistor closer to the state of punch-through or break-down, and this is never desirable.